Clinical and Experimental Evidence of Supernormal Excitability and Conduction

Abstract

True supernormality of excitability and conduction has been demonstrated in normal Purkinje fibers in in vitro studies. In the clinical setting, supernormality of conduction is manifested better than expected. This phenomenon is much more common than previously thought, particularly in the presence of certain clinical conditions. If a careful scanning of the cardiac cycle is performed on all patients with intermittent bundle branch block and second degree or advanced infranodal AV block, accessory pathways and mulfunctioning pacemakers, it is anticipated that a much larger amount of supernormal excitability and conduction will be unmasked.

Transient high‐degree right bundle branch block masking the type 1 Brugada ECG pattern associated with possible transient early repolarization syndrome

“…The presence of a rather long relative refractory period of specialized fibers would facilitate the existence of a well‐developed supernormal phase and eventually supernormal conduction. In fact, supernormal excitability implies that conduction of the cardiac impulse may improve during such a period (Elizari, Schmidberg, Atienza, Paredes, & Chiale, ), particularly when the phenomenon is systematically searched for in patients with advanced AV block or intermittent conduction in the His‐Purkinje system or in the case of accessory pathways. One of the main unresolved issues is to elucidate the exact mechanism of this almost forgotten electrophysiologic curiousness.…”

QRS normalization during atrial pacing in a patient with complete left bundle branch block: What is your diagnosis?

“…However, atrial extrastimulation was associated with LBBB morphology for H-H1 <440 ms. The long duration of H-H1 value (286 ms) associated with narrow QRS complexes is unlikely to represent the duration of the “supernormal phase of conduction,” since such period has been reported to occur only during a short time period ranging from 10 to 90 ms 1 . We should recognize, however, that the conduction system in our patient is not in a steady state and that previous studies on supernormal activity may or may not apply to this case, making it possible that the supernormal period may be much longer.…”

“…A third possibility is supernormal conduction of the left bundle branch (LBB). A supernormal phase of conduction has been defined as a “condition under which conduction improves and occurs in relation to a supernormal phase of excitability in a sick tissue.” 1 Supernormal conduction was described in cases of intermittent BBB. The patient’s bundle recording, along with programmed atrial stimulation with a full scan of the diastolic interval, is an essential requirement for the recognition of a supernormal phase of conduction in order to exclude other confusing explanations.…”

“…Normalization of conduction is preceded and followed by impaired conduction in the bundle branch, indicating that the supernormal phase is immersed in the recovery curve of the affected fascicle without changes in His-Purkinje conduction times (H-V). Elizari and colleagues 1 analyzed 20 cases of supernormal conduction in the setting of tachycardia-dependent block (phase 3 BBB). They found that the duration of the refractory period ranged between 600 and 1020 ms.…”

“…The duration of the supernormal phase extended from 10 to 90 ms and its position in the cardiac cycle was relatively constant. 1 …”